The present invention relates to ultraviolet-autocurable oligomers. More particularly it relates to acrylo-urethane oligomers having built-in benzophenone type photo-initiators, which are auto-curable through ultraviolet (UV) light radiation.
Since the development of high intensive radiation processing such as ultraviolet (UV) light and electron beams as an alternative to heat processing, photo-curable resins are now widely used for coating, printing, adhesion, and photo-imaging. In general, for fast and efficient curing of such resins, it is necessary to include a relative large amount of the initiating component within the system. However, such levels of photo-initiator can not be used in certain system where high levels of photo-initiator do not dissolve or mix homogenously.
U.S. Pat. No. 4,004,998 discloses a photo-polymerizable compound having built-in sensitizers which is auto-polymerizable through UV radiation. Since the photo-initiators are incorporate within the polymerizable system through a chemical reaction between the photo-initiator and the polymerizable monomer, the compatibility problem that results by merely physically mixing the photo-initiators with the polymerizable monomers is prevented. The photo-polymerizable compound disclosed in the above U.S. Patent comprises the product of the reaction of a monomeric hydroxy-containing polyethylenically unsaturated ester or a monomeric isocyanate-modified hydroxy-containing polyethylenically unsaturated ester and a monocarboxy-substituted benzophenone, e.g. benzophenone tetracarboxylic dianhydride (BTDA). However, in Example 19 thereof the monoethylenically unsaturated esters having free hrdroxyl groups were demonstrated to be particularly not acceptable for preparing the photopolymerizable product because the speed of curing was too slow, the viscosity of the solution was too high, and the surface properties were poor. As well known to those skilled in the art, the polymeric coating prepared from a monomeric compound will have a poorer surface properties compared to the one prepared from a resin.
U.S. Pat. No. 3,759,809 discloses an isocyanate-modified polyethylenically unsaturated ester having free hydroxyl groups, which is mixed with a photo-initiator to provide a photopolymerizable composition having reduced water sensitivity and reduced toxicity. This isocyanate-modified polyethylenically unsaturated ester is a reaction product of a polyethylenically unsaturated ester having free hydroxyl groups formed by the reaction of an ethylenically unsaturated acid and a polyhydric alcohol with an organic isocyanate. However, the inventors in Example 18 demonstrated that an isocyanate-modified product, where the starting ester was monoethylenically unsaturated, was not accepted because the speed of curing was too slow, the viscosity of the solution was too high, and the surface properties were poor. Furthermore, in Example 19 an isocyanate-modified product, where the starting ester was reacted with sufficient phenyl isocyanate to convert all of the hydroxyl groups to carbamate groups, was found not acceptable as a lithographic ink vehicle because of stripping. Basically, the composition of this U.S. Pat. No. 3,759,809 still suffers the conventional incompatibility problem between the resin and the photo-initiator and instability in storage.